The use of copper metallization in semiconductor devices is becoming more preferred as a replacement for aluminum. The lower resistivity of copper versus aluminum (<2 μΩ-cm versus >3 μΩ-cm) enables smaller line widths and depths. Thinner metal lines also reduce capacitance between lines, reducing overall power consumption. Because copper is difficult to dry etch, damascene or dual-damascene integration schemes are preferred.
FIG. 1 illustrates a portion of a semiconductor wafer 10 comprising a substrate 12, a dielectric layer 14 such as silicon dioxide (SiO2), phosphosilicate glass (PSG), borosilicate glass (BSG), and borophosphosilicate glass (BPSG), with vias/trenches 16 etched into the dielectric layer 14. A thin layer 18 of a barrier material (e.g., tantalum, titanium, or titanium nitride) is deposited over the dielectric layer and into the openings 16. A copper layer 20 is then deposited over the barrier layer 18 by conventional methods such as chemical vapor deposition (CVD), physical vapor deposition (PVD), and electroplating, to fill the openings 16.
To electrically isolate the copper interconnects, excess copper layer 20 and barrier layer 18 is removed, typically by chemical mechanical polishing or planarization (CMP), as shown in FIG. 2. CMP processes are abrasive techniques that are frequently used to remove material from or planarize the surface layers of a wafer during fabrication of integrated circuit devices. Typically, a wafer is pressed against a polishing pad in a slurry solution under controlled conditions. The slurry solution generally contains abrasive particles that mechanically remove the surface layer and may contain chemical agents, such as an oxidant such as hydrogen peroxide, and abrasive particles such as aluminum oxide (alumina), titanium dioxide, silicon dioxide, and/or cerium dioxide, that abrade the surface layer. Planarizing the wafer produces residual particles 24 including metal particle accumulations, residual abrasive slurry particles, and other types of residual particles that are generally loose and unattached to the planarized surface 26. Surface attraction can vary, and some particles may be embedded in the surface 26.
After a CMP process, the planarized surface layer 26 is then cleaned to remove residual chemicals and particles that remain on the wafer surface. Post-planarization cleaning can be performed using various methods depending on the composition of the layer and any residual chemicals and particles that may be present. The cleaning methods are generally wet cleaning processes that include chemical cleaning, mechanical scrubbing, and other surface agitation techniques. The objective in the cleaning process is to provide a wafer surface that is free of slurry particles, organic residues and trace metals ions, without introducing defects to the wafer surface.
The current technology for metal CMP post-cleaning uses mechanical brush scrubbers in conjunction with deionized water and/or cleaning solutions, and/or immersion in a vibrational bath. In an exemplary process, the CMP wafers are rinsed, immersed in a megasonics bath, and then subjected to double-sided brush scrubbing using a cleaning solution to scrub both sides of the wafer.
Cleaning wafers after copper CMP presents various challenges, including issues of corrosion and particle removal that must be addressed. Aluminum CMP post-cleans that utilize ozonated deionized (DI) water or other oxidizing media passivation between polish and post-cleans, are not functional for a copper CMP post-clean because of high copper corrosion rates in such media. Processing of copper CMP wafers using a low-pH aluminum scrub leaves high levels of alumina slurry residuals, which can cause problems such as brush loading (i.e., adherence of alumina particles to the brush). In addition, exposure to DI water must be minimized for copper CMP polished lots to prevent deionized (DI) water corrosion.
A cleaning solution comprising dilute ammonium citrate can be used for copper CMP post-clean processes. One example of a copper CMP cleaning solution comprises water, an ammonium salt of a hydroxycarboxylic acid such as ammonium citrate, a dicarboxylic acid such as oxalic acid and malonic acid, and optionally a small amount of ammonia fluoride or hydrofluoric acid, in an acidic pH of about 3.0 to about 6.0. Ammonium citrate solutions have been found to disperse alumina and silica slurry particles, and do not promote corrosion of copper. In addition, such solutions are relatively inexpensive.
However, a drawback of ammonium citrate cleaning solutions is their nutritional appeal to certain strains of bacteria, for example, Bacillus sp. Cleaning agents such as ammonium citrate can support microbial activity and/or growth in solution. If present in the cleaning media, such bacteria can be carried to the wafer during processing. Once there, biosorption of bacteria to the copper surface can make removal of the bacteria very difficult. Such bacterial contamination can be detected, for example, by SEM inspection.
One potential technique to remove bacterial contamination is to remove organic contaminants with a peroxide flush of the system. However, residual peroxide in the lines can result in copper erosion.
Therefore, it would be desirable to provide a cleaning composition that inhibits growth of bacteria in the solution without hampering the process performance of the solution.